Certain mechanical devices are now becoming miniaturized to such an extent that forming their components from traditional materials such as metals presents severe manufacturing problems. The semiconductor industry has developed many techniques for forming very small electronic components, such as transistors, in single-crystalline silicon at a very low cost per component. In Silicon Micromachined SCALED Technology, 42 IEEE TRANS. ON INDUSTRIAL MICROELECTRONICS, No. 3, 234-239 (1995), Denny K. Miu and Yu-Chong Tai describe fabricating miniature flexible mechanical devices from single-crystal silicon. Those used to thinking of single-crystal silicon as a hard, brittle substance might regard single-crystal silicon as an unlikely choice of material for making flexible mechanical devices. However, single-crystal silicon has a Young's modulus almost equal to that of steel. Single-crystal silicon about 40 .mu.m (0.0015 mil.) thick is flexible and springy. Single-crystal silicon differs from steel in that it breaks when stressed beyond its ultimate strength instead of yielding. The main attraction of substituting single-crystal silicon for metals in fabricating flexible miniature mechanical devices is that the semiconductor industry's manufacturing techniques can be adapted to mass produce such devices at low cost.
An example of a flexible mechanical device that can be made with advantage from single-crystal silicon is the gimbal used to mount the record/playback head in the actuator arm in a hard disc drive. The rapidly-increasing data density demanded from such disc drives results in progressive reductions in the size of the head and the actuator arm, and, hence, in the gimbal. A requirement exists for gimbals having a maximum linear dimension of less than 1 mm and a thickness measured in tens of microns. The gimbal must mount the head on the actuator arm in a way that allows the actuator arm to move the head rapidly parallel to the surface of the disc, and that also allows the head to float on a film of air a few tens of nanometers thick between the head and the surface of the disc.
Typical micromechanical devices, such as gimbals, are difficult to make using a process that includes wafer handling for conventional-thickness wafers because of their extreme thinness, typically less than 20 .mu.m. As noted above, single-crystal silicon wafers of this thickness are floppy and fragile, whereas wafer handling for conventional-thickness wafers is designed to operate with wafers that are rigid and over approximately 450 .mu.m thick. The floppiness and fragility of 20 .mu.m wafers makes such wafers impossible to process using wafer handling for conventional-thickness wafers.
Miu and Tai (above) describe a way of overcoming this problem by fabricating very thin micromechanical devices, gimbals, in conventional-thickness single-crystal silicon wafers. Part-way through the fabrication process, the back side of the conventional-thickness wafer is masked with a potassium hydroxide (KOH)-resistant mask, apertures positioned under each micromechanical device are formed in the mask, and the wafer is subject to an anisotropic back etch in which the wafer is etched from its back side using a KOH etchant. The etching process forms a deep pit under each micromechanical device in the conventional-thickness wafer. Each pit is capped by a membrane of the wafer that includes one micromechanical device.
The technique of performing a timed anisotropic etch from the back side of the wafer has several disadvantages, even though it enables very thin micromechanical devices to be produced by processing conventional-thickness silicon wafers using wafer handling for conventional-thickness wafers. First, the process is very wasteful of the area of the silicon wafer. The angle of the side walls of the pits etched in a &lt;100&gt; wafer is about 54.7.degree. to the surface of the wafer. The dimensions of typical current micromechanical devices are of the same order as the thickness t of a conventional-thickness wafer. Thus, as can be seen in FIG. 1, the width w.sub.w of the wafer 1 required to accommodate the micromechanical device 3 is over twice the width w.sub.g of the micromechanical device itself. The area of the wafer effectively occupied by each micromechanical device is more than three times the area of the micromechanical device. The large effective area of each micromechanical device reduces the number of micromechanical devices of a given size that can be fit on a silicon wafer of a given size by a factor of the order of three. Making so few micromechanical devices on each wafer increases the manufacturing cost of each micromechanical device, because the cost of processing the wafer is divided among significantly fewer micromechanical devices.
A second disadvantage of performing a timed anisotropic etch from the back side of the wafer is that the etch removes a large amount of material from the conventional-thickness wafer, and leaves the conventional-thickness wafer in a weakened condition. Consequently, the process is preferably performed towards the end of the process of fabricating the micromechanical devices. However, no KOH-incompatible elements, such as aluminum lead patterns, can be included in the micromechanical devices before the KOH back etch. This means that at least part of the fabrication of the micromechanical devices must be performed with the wafer in its weakened state, with consequent steps being required to reduce the risk of the weakened wafer breaking.
Finally, a timed anisotropic etch performed from the back side of the wafer can result in micromechanical devices having poor thickness uniformity, and having a rough back surface. A rough back surface promotes stress cracking, which can impair the reliability of the micromechanical devices.
Accordingly, an improved method for making thin micromechanical devices in single-crystal silicon using processing including wafer handling for conventional-thickness wafers is required. The method must be mechanically compatible with semiconductor fabrication processing that includes wafer handling for conventional-thickness wafers, and must enable many more micromechanical devices to be made on each wafer to reduce the manufacturing cost per device. The method must also reduce the amount that weakened or fragile wafers have to be handled during the production process, and must produce micromechanical devices with a superior thickness uniformity and that are not subject to stress cracking.